What advice do you have for a prior-service Marine entering the Army?

Prior Marines generally either do exceptionally well, or exceptionally poor. The best thing you can do is immediately remove any chips that remain on your shoulders, forget all of the Marines "we're better than everyone" brainwashing nonsense, and get ready to adapt to your new environment. Listen a lot. Learn a lot. Talk very little. At least at first.
Start reading Army Regulations. Start with uniforms, awards, customs and courtesies, NCO guides, etc...
It's gonna take you time to get your records straight, so be prepared to be patient. Most S1's aren't used to prior Marines, so you getting your awards, military education, NCOES, etc... straight will be an adventure. Stay patient.
Don't rush towards promotion because you think all of your Marine experience makes you ready to be an Army NCO. It doesn't. Before you rush to the board, make sure you truly understand Army basics, counseling, awards, evaluations, promotions, your MOS, service and programs, uniforms, leaves and passes, all those thousands of things NCO's are expected to know. Remember, it's better to be a high speed E4 than a POS E5. Make sure you're ready to be an Army NCO.
This might seem negative in tone. It's not. I made the same transition and it was the best thing I ever did. In the Army, you'll have a better quality of life, better programs, more education, more training, more varied responsibilities and assignments, more diverse bases and units, and a real opportunity to shine. But remember, you joined the Army. You must adapt. If you don't succeed, the Army will replace you faster than you can walk out the door. Make sure you set yourself up for success. Find another prior Marine, preferably a Senior NCO or Officer for mentorship, guidance and advice. They'll understand the transition. Don't latch on to a junior prior Marine. It remains to be seen whether they'll sink or float.

I liked working with former Marines. They were always more even keeled and reserved with their emotional reactions. They got their tasks done without griping, and almost all of them kept Soldiers motivated without blowing up.

Prepare to slow down (so to speak). I have worked with the Marines during my time in and most of the time they seem to be to perfect, that being said never loose what you learned in the Marines use it to your best advantage. You have already learned basic Soldiering skills so now you have to learn how to use them to your advantage. PT is going to be one of the big Little things Exercise are different but produce the same results, Put on your learning head gear everyone you meet is someone to learn from. Now watch those Junior Soldiers that want to help to much most have different idea's of what works the best. Stick with your Staff NCO's and never be afraid to ask a question. Marines Find the Army a little lax, I guess is what I am trying to say so use your High speed low Drag energy and learning traits to enhance your Career. Give yourself plenty of time to learn before worrying about Promotion. Never discard what you learned in the Marines just put it in a different folder so you can pull it out for use later. Good Luck and welcome to the Army.

The following are some cultural faux pas I've observed committed by former Marines who joined the Army:
1. The statement "Well, in the Corps we did it like this ..."
~ You are more than welcome to continue doing it that way in the Corps or submit through channels a request to change SOP, policy, regulation, etc. This is like telling your current spouse/girlfriend how much better your ex-spouse/girlfriend in the sack. Yes, I agree the USMC's shooting regimen and PT and definitely, uniforms are so much better than the Army's but, don't tell your current spouse/girlfriend how great your ex kept herself in such great shape.

2. The statement "The Corps is so much better than the Army because ..."
~ Along similar vein as above but this is an overarching sentiment stating how your ex-spouse/girlfriend is better overall than your current spouse/girlfriend.

3. Understand the Army has four titles for NCOs:
~~ Corporal - Self-explanatory.
~~ Sergeants - Refer to all sergeants, staff sergeants, sergeants first class and master sergeants as "Sergeant". Also, when you get promoted to SSG, SFC or MSG, don't get tweaked out because someone called you Sergeant.
~~ First Sergeant - Self-explanatory.
~~ Sergeant Major - Refer to Sergeants Major and Command Sergeants Major as "Sergeant Major"

4. We capitalize all our abbreviations for our ranks and keep it to three characters:
PV1 ... SPC/CPL ... SFC
PV2 ... SGT ... MSG/1SG
PFC ... SSG ... SGM/CSM/SMA

5. D&C -
~~ We have "Rear MARCH" not "To The Rear, MARCH"
~~ Left as in Left Face, has one syllable so try to limit the LEY-YEFTs .

6. Calling cadence. By far the most humorous to me is listening to a former Marine calling cadence in a gravelly muffled voice that many in formation did not understand. That said, God bless him for trying.

You will always be a Marine but as the saying goes, "When in Rome do as the Romans"

I was an enlisted Marine (3381) prior to commissioning in the army. There are pros and cons to each branch. I'll list a few but feel free to message me if you have specific questions.

Army Pros: Better equipment and better facilities leads to better training. The Army has more opportunities to go to schools and tryout for things like SF if you were so inclined. More Army bases means you can be stationed anywhere in the country or the world. Whereas with the Marines you're basically limited to east coast, west coast, and Okinawa (with a few other exceptions).

Army cons: Once you get outside of the Infantry (and probably Combat Arms in general) in the Army, esprit de corps and discipline take a plunge downward as compared to the Marines. As you know, in the Marines everyone is held to the same high standard. The Army is not that way. People can argue that but I've been on both sides and that's just the way it is. That's not to say that the soldiers in those areas aren't great at what they do, you will just notice a lack of intensity that's found in the Corps. Soldiers in the Army, while proud, will not have the same level of pride as Marines. I just came from an Army school and at the graduation, not everyone knew all the words to the Army song. This would NEVER happen in the Marines. And if it did, that guy better watch out.

Overall though, I've enjoyed my time in the Army more than the Marines. Maybe it's personality or something else. I've just found that it fit me better.

SFC Hasbun hit it right on point!! Your Army Regulations will definitely help with your transition. As far as careers, I’m a 42A and I LOVE my job!! It is very rewarding and offers a lot of opportunities to receive additional training throughout your career as well as awesome assignments. It’s very easy to network in the 42A arena because with work with all MOSs. As far as promotions, there is still a required time in service to meet when you come in. Although you’re prior service, there is still a time in grade requirement you have to meet. I wish you the best of luck in your future endeavors and keep it touch if you ever have questions.

I'd say 42, but i am biased

Oorah Brother! I'm prior service Marine as well. I have 2 years in the Army so far. I got out Honorably and joined the Army 7 years later. Being prior service I was told I wouldn't have choices like when I first joined the Marines. I was only given one option and it was a take it or leave it, because I was prior service. I kept my Cpl rank when I joined. I ended up joining the Army Reserves, because at that time only prior service Special Ops or prior service with a GT score of 110 or more could go active duty. They told me there was no waiver for my GT score of 107... like I said, it was take it or leave it... I signed and took my oath on a Saturday and then on Monday I reported to my new unit. Yes, on the Momday of that same weekend... It took my unit about 4 to 5 months to get my service uniforms. I've been in for 2 years and I still have yet to get my dress uniform... I was sent to my mos school, but instead of it being 13 weeks like a new Soldier, it was a 4 weeks crash course with an exam every week. It took me a year and a half to get promoted to Sgt. You're going to have to complete SSD1 (online modules) and complete Basic Leadership Course (BLC) before you can get promoted to SGT. A lot of the Marine Corps things wont transfer over, except for your medals. I can't wear a deployment patch on my uniform, because I want deployed while in the Army. Even though a did a tour to Iraq in the Marines. (And yes, I looked up the regulations). I am now awaiting Active Guard Reserve orders. (It's like active duty, but better). My original intention was to go Active, but my recruiter failed to mention the requirements for prior service trying to go active (ie. prior service special ops or GT score of 110 or higher)... The Army, from what I have experienced in the past 2 years, is way more laxed, than the Corps. There are times, when I get sick to my stomach because of the level of laxed discipline I've seen.. And unlike the Corps., Soldiers will get their feelings hurt when you yell at them, lmao. There are very few that have thick skin. The only one that understands the way I think, is a brother who was prior Army Infantry, before he changed MOS and joined my Army Reserves unit. Besides that, I wasn't sent to refresher course to learn about the Army Ways, like my recruiter said I would. I've had to learn by reading regulations and asking questions and going to BLC. There are still many things I dont kmow. As a Marine hold on to that pride that was instilled into us. But keep an open mind and keep yourself calm. Listen and learn. Marines always adapt and overcome. So you will do fine. PLEASE, dont start stuffing your cover into your trouser pockets! (You'll see...lmao..) It's going to difinetley be different, but you're already one step ahead, compared to those Soldiers that have been in 4 years or less. There are many things Soldiers dont learn that all Marines are expected to know. Just don't get carried away. Continue to learn and be more patient than you are as a Marine. Learn the Army ways and don't get too lax. You'll do just fine. Good luck!!

Additionally the NG is offering a bonus of 50k paying into college for OCS which is a huge bonus for me...

I made the switch to the Army Reserves and have no complaints. Don't really over think it to much the Army is still the military and most of what you know holds true. The Army has some great NCO's find some in your unit and learn all you can.

Read over AR 670-1 and go check out some youtube videos of army PRT. Practice the sit ups they are harder then you think. You can check out some Army DNC videos as well. The Army does things a little different then you will be used to.

Good luck on your studies, completing your degree will give you a jump on higher Enlisted promotions, and will open doors for you as well for Warrant Officer and Officer positions as well.